Substantially-uniform-temperature annealing

ABSTRACT

A system for heating optical members includes a thermally-conductive inner housing defining an interior volume for receiving an optical member to be heated, a thermally-insulative outer housing at least partially containing the thermally-conductive inner housing, and a heating structure disposed outside the inner housing and configured to provide heat through the thermally-conductive inner housing and into the interior volume defined by the inner housing.

FIELD OF THE INVENTION

The invention relates to annealing and more particularly to annealing ofsingle crystals to yield single crystals with low stress birefringencesuch as for use as optical lenses.

BACKGROUND OF THE INVENTION

The increase in the processing speed, functionality, and integration inintegrated circuits (ICs) has been achieved through continuous reductionin the feature sizes of the ICs. A portion of the manufacturing of theICs affecting attainable feature sizes is photolithography. Duringphotolithography, a pattern of the IC is transferred from a mask to awafer, e.g., a semiconducting wafer. Imaging characteristics of modernprojection optical photolithography equipment are dominated bydiffraction effects. The resolution (i.e. the smallest feature size thatcan be printed on the wafer) is k₁ λ/NA, where λ is the wavelength ofthe light source, k₁ is a constant approximately equal to 0.5, and NA isthe numerical aperture of the projection optics. The depth of focus ofthe projection printer over which the image quality is not degraded islimited and is equal to k₂ λ/(NA)², where k₂ is a constant that dependson k₁. Thus, to decrease the feature size either the wavelength ofexposure must be reduced or the NA of the optics must be increased.

Increasing the optics NA to reduce feature size results in a substantialreduction in the depth of focus (˜(NA)⁻²), which is undesirable,particularly because the depth of focus must be larger than anyvariations in the flatness of the photoresist surface. Therefore, thesemiconductor industry is pursuing the use of short wavelength exposuresources for achieving smaller and smaller feature sizes. KrF, ArF, andF₂ excimer lasers are presently available as light sources for,respectively, 248, 193, and 157 nm photolithography. The synthetic fusedsilica, however, that has been the optical material of choice for higherwavelength exposure sources, exhibits significant loss of transmittanceat wavelengths below 200 nm.

Single crystals of Calcium Fluoride (CaF₂) exhibit the desirable opticalproperties for sub 200-nm-photolithography. Furthermore, for historicalreasons the production knowledgebase for CaF₂ is relatively extensive.Other single crystals of fluoride such as BaF₂ and LiF are also possiblematerial candidates, but are significantly behind CaF₂ in productiontechnology, and may be less desirable, e.g., due to toxicity andcorrosiveness (BaF₂) and/or expense (LiF). Therefore, single crystalCaF₂ are desirable and suitable optical material for 193 and 157 nmoptical steppers. Presently, CaF₂ crystals as large as 30 cm in diameterand 10 cm in height are used in photolithography equipment.

Single crystals of CaF₂ are grown by directional solidification from themelt phase. In this process layers of the melt are continuouslysolidified, by changing the temperature of the crystal, to form a singlecrystal boule. The crystal boule is subsequently cooled to roomtemperature. The transfer of heat from and through the crystal sets uptemperature gradients (i.e. temperature non-uniformities) and associatedthermal stresses in the single crystal. CaF₂ is a relatively weakmaterial, especially at elevated temperatures, and therefore experiencesplastic deformation under thermal stresses during the crystal growthprocess. The accumulation of plastic strain during the crystal growthprocess results in generation of residual stresses in the crystal atroom temperature. Residual stresses, in turn, cause stress birefringencethrough spatial variations in the material's index of refraction, and anassociated degradation of optical characteristics of components madefrom this material.

Annealing is used to reduce residual stresses in crystals that haveexperienced plastic deformation during the crystals' growth process. Toanneal a crystal, the crystal is maintained at an elevated temperatureclose to its melting point temperature for a period of time. Thisconstant temperature is intended to allow existing residual stresses torelax. The crystal is cooled to room temperature. During cooling,temperature gradients associated with the cooling of the crystalgenerate thermal stresses in the crystal that may cause the crystal toundergo plastic deformation.

Due to the nature of the material, temperature variations to which asingle crystal is exposed to during growth and annealing result in largethermal stresses leading to plastic deformation of the crystal and,hence, large residual birefringence.

SUMMARY OF THE INVENTION

In general, in an aspect, the invention provides a system for heatingoptical members. The system includes a thermally-conductive innerhousing defining an interior volume for receiving an optical member tobe heated, a thermally-insulative outer housing at least partiallycontaining the thermally-conductive inner housing, and a heatingstructure disposed outside the inner housing and configured to provideheat through the thermally-conductive inner housing and into theinterior volume defined by the inner housing.

Implementations of the invention may include one or more of thefollowing features. The inner housing is configured such that an innersurface defining the interior volume has a substantially uniformtemperature in response to the inner housing receiving the heat providedby the heating structure. The inner housing is configured to define theinterior volume to be axi-symmetric.

Further implementations of the invention may include one or more of thefollowing features. The system further comprises a controller coupled tothe heating structure and configured to control the heating structuresuch that the member disposed in the interior volume is heatedsubstantially without being plastically deformed. The controller isconfigured to control the heating structure such that a resolved shearstress of a CaF₂ optical member disposed in the interior volume does notexceed about 0.5 e^((990/T)) MPa where T is average temperature of themember in Kelvin.

Further implementations of the invention may include one or more of thefollowing features. A portion of the outer housing in contact with andsupporting the inner housing has a thermal conductivity different thanat least one other portion of the outer housing. An inner boundary ofthe outer housing is disposed in contact with substantially an entireouter boundary of the inner housing. The inner housing and at least aportion of the outer housing are an integral structure, with the innerhousing and the at least a portion of the outer housing being layers ofthe integral structure with different thermal conductivity.

Further implementations of the invention may include one or more of thefollowing features. The inner housing comprises at least one ofhigh-thermal-conductivity graphite and high-thermal-conductivity carbon.The interior volume is cylindrical and directions of highest thermalconductivity of the inner housing are parallel with inner surfaces ofthe inner housing. The interior volume is cylindrical and directions oflowest thermal conductivity of the inner housing are perpendicular withinner surfaces of the inner housing. Directions of lowest thermalconductivity of the outer housing are perpendicular with outer surfacesof the inner housing.

Further implementations of the invention may include one or more of thefollowing features. The inner housing has substantially orthotropicthermal conductivity. The outer housing comprises at least one oflow-thermal-conductivity graphite, low-thermal-conductivity carbon,low-thermal-conductivity porous graphite, low-thermal-conductivityporous carbon, low-thermal-conductivity fibrous graphite,low-thermal-conductivity fibrous carbon. The outer housing hassubstantially orthotropic thermal conductivity. The system furthercomprises another thermally-conductive housing, the anotherthermally-conductive housing substantially contains thethermally-insulative outer housing. The another thermally-conductivehousing is displaced from the outer housing.

Further implementations of the invention may include one or more of thefollowing features. The inner housing defines a plurality of interiorvolumes each for receiving an optical member to be heated. The innerhousing has a substantially isotropic thermal conductivity. The outerhousing has a substantially isotropic thermal conductivity. At least aportion of the heating structure is disposed outside the outer housing.

In general, in another aspect, the invention provides a method ofheating an optical member. The method includes providing the opticalmember, directing heat from a heat source toward the optical member, anddistributing the heat about the optical member through ahigh-thermal-conductivity apparatus disposed between the heat source andthe optical member such that a surface of the apparatus defining avolume for receiving the optical member will have a substantiallyuniform temperature.

Implementations of the invention may include one or more of thefollowing features. The heat is distributed such that temperatures ofthe surface of the apparatus defining the volume vary by no more thanabout 0.5 K where K is temperature in Kelvin. The method furthercomprises measuring at least one indication of temperature of theapparatus defining the volume. The at least one indication includes aplurality of indicia of temperature of the apparatus, the indicia beingrelated to at least one of an outer surface, an inner surface, and aninterior of the apparatus. The method further comprises adjusting howmuch heat is directed toward the optical member in accordance with theat least one indication. The adjusting is in accordance with a model oftemperature variations within the optical member. How much heat isdirected toward the optical member is adjusted to guard against stresswithin the optical member exceeding a critical resolved shear stress ofthe optical member during at least one of annealing of the opticalmember and cool down of the optical member.

Further implementations of the invention may include one or more of thefollowing features. The method further comprises inhibiting heat fromtransferring away from the optical member from thehigh-thermal-conductivity apparatus. A plurality of optical members isprovided, wherein heat is directed from a heat source toward each of theoptical members, and wherein the heat is distributed about each of theoptical members through the high-thermal-conductivity apparatus disposedbetween the heat source and the optical members such that surfaces ofthe apparatus defining volumes for receiving the optical members willeach have a substantially uniform temperature.

In general, in another aspect, the invention provides a system forannealing at least one single crystal blank for use as at least oneoptical lens. The system includes a heating structure for supplyingheat, heating means for heating the at least one single crystal blank,using the heat from the heating structure, to an annealing temperatureof the blank and for cooling the at least one single crystal blank fromthe annealing temperature to an ambient temperature substantiallywithout plastic deformations developing in the at least one blank, theheating means including at least a high-thermal-conductivity housing forcontaining the at least one single crystal blank.

Implementations of the invention may include one or more of thefollowing features. The heating means further includes an insulatorstructure at least partially containing the high-thermal-conductivityhousing. The heating means further includes a controller coupled to theheating structure for regulating heat provided by the heating structureto permit annealing of the at least one blank while inhibitingtemperature gradients inside the at least one blank from producingplastic deformations. The heating means further comprises temperaturesensors coupled to the controller configured to provide indicia oftemperatures of the high-thermal-conductivity housing to the controllerand wherein the controller regulates the heat provided by the heatingstructure in response to the indicia provided by the temperaturesensors. The controller inhibits temperature gradients inside each ofthe at least one blank from producing stresses in excess of about 0.5e^((990/T)) MPa where T is average temperature of each blank in Kelvin.

In general, in another aspect, the invention provides an optical memberincluding a single crystal material substantially free of residualstress and having an optical birefringence of less than about 1 nm/cm.

Implementations of the invention may include one or more of thefollowing features. The single crystal material forms an optical lensblank. The single crystal material is a fluoride. The single crystalmaterial is CaF₂.

Various aspects of the invention may provide one or more of thefollowing advantages. A substantially isothermal environment may beprovided for members, such as optical blanks or lenses, to be annealed(during annealing and cool down), or otherwise heat treated. Temperaturenonuniformities along walls of a chamber containing a member to beheated can be reduced relative to prior systems. Heat loss through asupport structure for supporting a chamber to contain a member to beheated can be reduced relative to prior systems. Time-dependentvariations of temperature on an interior portion of a container of amember to be heated can be dampened relative to correspondingtime-dependent variations on an exterior portion of the container.Radial temperature variations within an axi-symmetric crystal can bekept below a level that would induce stresses exceeding a criticalresolved shear stress of the crystal during an annealing and/or cooldown period. Annealed items, e.g., optical members, can be produced withlow birefringence, e.g., less than about 1 nm/cm.

These and other advantages of the invention, along with the inventionitself, will be more fully understood after a review of the followingfigures, detailed description, and claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross-sectional view (with a blank shown incut-away) of an annealing system.

FIG. 2 is a graph of critical resolved shear stress vs. temperature.

FIG. 3 is a block flow diagram of an annealing process using the systemshown in FIG. 1.

FIG. 4 is a cross-sectional view (with blanks shown in cut-away) ofanother annealing system, for annealing multiple blanks currently.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, an annealing system 10 includes a single crystalblank 12 for, an inner housing 14, an outer housing 16, including a cap18 and a base 20, a support 22, including a platform 24 and shaft 26, aheating structure including heating elements 28, 30, 32, controllers 29,31, 33, temperature sensors 40, 42, 44, an insulator 36, and a systemhousing 38. The system 10 is configured for high-temperature annealingof single crystals as described below. The system 10 can accuratelymaintain temperature in the crystal 12 at levels equal to and belowlevels needed to promote complete, or near complete, relaxation ofstresses during a constant-temperature phase of an annealing process.The system 10 can also maintain spatial temperature non-uniformities inthe crystal 12 at or below levels inducing plastic deformation of thecrystal 12 and accumulation of plastic strain in the crystal 12 during acool-down phase, to room temperature of the annealing process.

The blank 12 is any material to be annealed. For example, here the blank12 is a single crystal of a material suitable for use as an opticalcomponent. One or more optical components may be made from the blank 12,e.g., by dividing such as by cutting the blank 12. For example, theblank 12 can be a single crystal fluoride material such as calciumfluoride CaF₂, although other materials may be used. The blank 12 can beaxi-symmetric, for example being cylindrical about an axis 46. Blank 12can be of various shapes and/or sizes, e.g., or cylinder, e.g., 30 cm indiameter and 10 cm in height.

The inner housing 14 is a high-thermal conductivity material configuredto contain the blank 12. The inner housing 14 defines an interior volume48 of a size able to receive the blank 12. A resealable entry (notshown) is provided in the housing 14 to be opened to receive the blank12 and to be sealed during annealing of the blank 12. The housing 14 ismade of a high-thermal conductivity material, such as high-puritygraphite EK 94P, made by Ringsdorff-Werke GmbH of Bonn, Germany. Thismaterial, according to the manufacturer, has a thermal conductivityclose to 0.7 W/cm-K at 1200 K. The housing 14 is configured such thatits top and bottom walls 15, 17 have their highest (lowest) thermalconductivity directions parallel (perpendicular) to a radial directionfrom the axis 46. Also, the side/lateral wall 19 has its highest(lowest) thermal conductivity direction parallel (perpendicular) to theaxis 46. Thus, the highest (lowest) thermal conductivity directions areparallel (perpendicular) to an interior surface 50 of the inner housing14. Thickness(es) of the housing 14 is (are) such that heat provided bythe heaters 28, 30, 32 to the housing 14 will be conducted anddistributed throughout the housing 14 to provide a substantially uniformtemperature on the interior surface 50 of the housing 14.

Adding to the ability of the system 10 to provide a substantiallyisothermal environment in the volume 48 is the outer housing 16. Thehousing 16 is insulative in nature, having a much lower thermalconductivity than the inner housing 14. The outer housing 16 provides ahigher thermal resistance in directions away from the inner housing 14(i.e., in directions normal to the exterior surfaces of the innerhousing 14) than the thermal resistance along the exterior surfaces ofthe inner housing 14. The outer housing 16 and in particular cap 18, isof a low thermal conductivity material such as graphite fiber foam, madeby Calcarb Limited of North Lanarkshire, Scotland. This material,according to Calcarb, has a thermal conductivity approximately equal to0.005 W/cm-K at 1273 K. The base 20 may contain some materials of aslightly higher thermal conductivity to increase stiffness of the baseto provide adequate support for the cap 18 and the inner housing 14,containing the blank 12. The cap 18 is shown in physical contact withthe inner housing 14, although a gap, such as a vacuum orinert-gas-filled gap, may be provided between the cap 18 and the housing14. The gas may include fluoride if a fluoride blank is used, or mayinclude argon.

The support 22 is configured to support the outer housing 16, containingthe inner housing 14, containing the blank 12, while providing thermalresistance. The support 22 is made of a hard material with the platform24 being of hard and soft (low thermal conductivity) materials in acombination such that the support 22 provides thermal resistance andsufficient rigidity to support the components shown. The shaft 26 of thesupport 22 extends away from the outer housing 16 through the insulator36 and the system housing 38.

The system housing 38 is a metallic housing defining an outer perimeterof the system 10. The housing 38 is configured to be sufficientlyair-tight and to allow for evacuation of gases from within the housing38 to produce pressures inside the housing 38 as low as one-tenth toone-hundredth of one atmosphere, as well as to allow introduction ofprocess gases of pressures up to slightly over one atmosphere. Thehousing 38 is water cooled to maintain a desired temperature,dissipating, as necessary, heat received from the heaters 28, 30, 32through the insulator 36. The outer housing 38 is supported by anexternal structure not shown in FIG. 1.

The insulator 36 is provided to help reduce heat loss from the system10. In particular, the insulator 36 is made of an insulating materialsuch as graphite to inhibit heat from the heaters 28, 30, 32 beingtransferred away from the blank 12.

The heaters 28, 30, 32 are configured to provide heat to heat the blank12 to desired temperatures for annealing, or other desired processes.The heaters 28, 30, 32, e.g., resistive graphite heaters, may beconfigured to directionally supply heat toward the blank 12. Heat fromthe heaters 28, 30, 32 may transfer in directions away from the blank12, and is inhibited from doing so by the insulator 36. The heaters 28,30, 32 are configured to supply amounts of heat in response to controlsignals received from respective controllers 29, 31, 33.

The controllers 29, 31, 33 are configured to send signals to the heaters28, 30, 32 to regulate the amount of heat provided by the heaters 28,30, 32 in response to temperature indicia provided by the temperaturesensors 40, 42, 44. The temperature sensors 40, 42, 44 monitor thetemperature at various points on the inner housing 14 (e.g., on exteriorsurfaces as shown, or on interior surfaces, or inside the housing 14)andprovide indicia of these temperatures through signals to the respectivecontrollers 29, 31, 33. The controllers 29, 31,33 use the temperatureindicia from the sensors 40, 42, 44, to provide the control signals tothe heaters 28, 30, 32 in accordance with temperatures or temperatureschedules, that depend on a particular process currently undergone bythe blank 12. The temperature and temperature schedules for the blank 12are determined in order to inhibit plastic deformations and residualstresses inducing stress birefingence in the blank 12.

Non-uniform temperature fields lead to thermal stresses in the crystal12, and excessive thermal stresses during growth and annealing causeplastic deformation of the crystal. The system 10 is configured toprovide post-growth annealing that maintains a quantifiably controllableuniform temperature distribution in optical members, such as fluoridecrystals, in particular CaF₂, both during the constant temperatureperiod as well as cool-down period of the annealing process.

In general, single crystals such as CaF₂ experience plastic deformationalong specific crystallographic planes and directions, the so-calledslip planes and slip directions. For example, the slip system of CaF₂ isdefined as {100}<110>, where {100} refers to the orientation of thefamily of vectors normal to the slip planes and <110> the family ofdirection vectors along which slip occurs.

The crystal 12 undergoes plastic deformation if the projection ofthermal stresses onto the slip directions, the so-called resolved shearstresses, exceed the so-called Critical Resolved Shear Stress (CRSS) ofthe crystal. The CRSS is a property of the crystal 12. Stresses higherthan the CRSS will result in plastic deformations and hencebirefringence. Stresses smaller than the CRSS will result in elasticdeformation of the material and will not cause permanent deformationsresulting from plastic deformation. Thus, stresses smaller than the CRSSwill not cause birefringence.

It has been concluded that the temperature dependence of the CRSS for asingle crystal of CaF₂ is given by:

CRSS=0.5 e^((990/T))  (1),

where T is the temperature in units of Kelvin, and the CRSS has theunits of MPa. Referring to FIG. 2, the CRSS 52 for CaF₂ according toEquation (1) is shown to decrease with increasing temperature, and viceversa. Although the CRSS of CaF₂ increases with decreasing temperature,it is fairly low even at temperatures close to room temperature. Thus,to help avoid plastic deformation, and hence birefringence, thetemperature variations in the crystal are controlled by the system 10.

For cylindrical blanks 12, radial temperature gradients are the primarymechanism for generation of thermal stresses in the single crystal blank12. Thus, stresses can be kept below the CRSS by controlling theedge-to-center radial temperature difference within the crystal, theradial temperature difference ΔT. ΔT can be approximated according to:$\begin{matrix}{{{\Delta \quad T} = \frac{CRSS}{{\varphi\lambda}\quad E}},} & (2)\end{matrix}$

where φ is a configuration number related to the slip system of thecrystal, λ is the thermal expansion coefficient of the material, and Eis the Young Modulus.

It has been calculated that for a cylindrical single crystal of CaF₂,regardless of the crystal's dimensions, radial temperature differences,ΔT, exceeding:

approximately 0.5° C. at 1000° C.,

approximately 0.7° C. at 800° C.,

approximately 1.2° C. at 500° C., and

approximately 3.5° C. at 200° C.,

will cause the CRSS to be exceeded. Thus, to avoid plastic deformationsinside a single crystal of CaF₂, even as large as 30 cm in diameter and10 cm in height, the system 10 is configured to keep the values of ΔT inthe crystal at relatively low values.

The temperature difference ΔT is proportional to the cooling rate forthe blank 12. The proportionality depends on the blank's materialproperties, size, and shape, and can be determined, e.g., by computermodels or analytical expressions (in simple cases). Using knowledge ofthis proportionality, Equation (1), and Equation (2), the cooling ratecan be determined to inhibit, if not prevent, plastic deformation of theblank 12. The invention provides a schedule for controlled cool down ofthe annealed blank 12 such that the CRSS is not exceeded. A schedule forcool down helps ensure that during removal of heat from the crystal 12during cool down, temperature variations in the crystal 12 aremaintained at such low values that plastic deformation of the crystal 12does not occur, or occurs within acceptable amounts.

Based on the properties of CaF₂ available publicly, a computationalmodel has been used to calculate the rate of cooling so as not to havethe center-to-edge temperature difference in a crystal induce stressabove the CRSS. For example, if the annealed part is a single crystal ofCaF₂ of diameter of 30 cm and height of 15 cm, and the entire surface ofthe crystal is maintained at substantially the same surface temperature,the surface temperature should obey the following cooling ratesschedule:

approximately 0.4° C./hr from about 995° C. to about 797° C.,

approximately 0.6° C./hr from about 797° C. to about 600° C.,

approximately 0.9° C./hr from about 600° C. to about 400° C.,

approximately 1.4° C./hr from about 400° C. to about 287° C., and

approximately 2.7° C./hr from about 287° C. and lower.

It has been concluded that the cooling rate for a cylindrically-shapedsingle crystal of CaF₂, with flat top and bottom, may be calculated fromthe formula: $\begin{matrix}{{{cooling}\quad {rate}} = {\frac{\Delta \quad {T \cdot {thermal}}\quad {diffusitivity}}{{{constant} \cdot {surface}}\quad {area}}.}} & (3)\end{matrix}$

The constant has a value close to 5.5 and can be determinedexperimentally or from numerical simulations for different shapes of theannealed part and various configurations of the invention, including butnot limited to, the system 10. The cooling rate is the cool-down rate in°C./sec. Surface area is the surface area of the annealed part in unitsof cm². Thermal diffusivity is a property of the annealed part in unitsof cm²/sec.

Referring to FIG. 3, with further reference to FIGS. 1-2, a process 60of annealing the blank 12 includes stages 62, 64, and 66. At stage 62the blank 12 is provided. At this stage, the blank 12 is placed in thevolume 48 defined by the inner housing 14.

At stage 64, the controllers 29, 31, 33 control the heaters 28, 30, 32to provide heat. The heaters 28, 30, 32 provide heat to heat the blank12 to a desired constant temperature for the constant-temperature phaseof the annealing process. The temperature of the blank 12 is attemptedto be kept at a constant and substantially uniform temperature by thecontrollers 29, 31, 33 receiving indicia of temperatures from thetemperature sensors 40, 42, 44 and providing control signals to theheaters 28, 30, 32. The control signals control (including causingvariances in, as appropriate) the power used by the heaters 28, 30, 32,and thus the heat produced by these heaters 28, 30, 32 as appropriate tomaintain the temperature of the surface 50 of the inner housing 14 andthereby the temperature of the blank 12. The power of the heaters 28,30, 32 is regulated such that the temperature sensors 40, 42, 44 measuresubstantially fixed set values. These values are held substantiallyconstant for the time duration of the constant-temperature phase of theannealing process.

At stage 66, the controllers 29, 31,33 regulate the heaters 28, 30, 32to cool the blank 12 down. Again, responsive to temperatures indicatedby the temperature sensors 40, 42, 44, the controllers 29, 31, 33 sendcontrol signals to the heaters 28, 30, 32 to adjust as necessary, thepower used and thus the heat provided by the heaters 28, 30, 32. Thecontrollers 29, 31, 33 regulate the heat provided such that thetemperature as indicated by the sensors 40, 42, 44 follow apredetermined cooling rate schedule that has been determined to guardagainst temperature gradients within the blank 12 causing stresses toexceed the CRSS of the blank 12. In particular, the heat is regulated toguard against temperature gradients in the blank 12 (e.g., radialtemperature gradients for a cylindrical blank 12) exceeding values thatwould cause resolved sheer stresses in the blank 12 to exceed the CRSSof the blank 12.

Using the method 60, the blank 12 can be produced having desiredcharacteristics. For example, the CaF₂ blank 12 can be produced withresidual stress birefringence that is less than approximately 1 nm/cm.This birefringence is then acceptable for very fine resolutionphotolithography applications.

Referring to FIG. 4, a system 70 similar to the system 10 (FIG. 1)includes components that are different from, but similar to, that ofsystem 10 to accommodate an inner housing 72 that is different from theinner housing 14 (FIG. 1). The inner housing 72 defines three volumes74, 76, 78 that are sized to accommodate three respective blanks 80, 82,84. Each of the blanks 80, 82, 84 may be the same or different sizes,e.g. cylindrical with a height of 10 cm and a diameter of 30 cm. Othercomponents of the system 70 are similar to the respective components ofsystem 10, but different in order to accommodate the multiple blanks 80,82, 84 while providing similar functionality, e.g., substantiallyisothermal environments for the blanks 80, 82, 84. While three volumes74, 76, 78 are shown in FIG. 4, other numbers of volumes may beprovided.

Other embodiments are within the scope and spirit of the appendedclaims. For example, referring to FIG. 1, a support structure can beprovided in the volume 48 defined by the inner housing 14 to separatethe blank 12 from the walls of the housing 14. Also, the housings 14 and16 may be integrally formed of layers having different thermalconductivities, with a higher thermal conductivity layer, or layers,being disposed inward of a lower thermal conductivity layer, or layers.The blank 12 may be of a variety of materials, such as semiconductors,or other materials, even if not for optical uses, for which annealing orother heating/cooling is desired and in which temperature gradients areundesirable. Also, items other than blank 12 can be annealed using thesystem 10, such as optical components, lenses, prisms, and singlecrystals, e.g., fluorides, other than CaF₂. Also, although each heaterand temperature sensor combination is shown in FIG. 1 with its owncontroller, a controller may be used to regulate more than one, and evenall, of the heaters responsive to temperature indicia from thetemperature sensors. The base 20 can be made of layers or can be acomposite of low and high thermal conductivity materials in order toprovide a sufficiently sturdy and sufficiently low thermal conductivitymember. One or more of the heaters can be enclosed in portions of thehousing 16 or support 22 (e.g., in platform 24). Temperature indiciaprovided by the temperature sensors 40, 42, 44 can be of the innersurface 50 of the inner housing 14, or an interior portion of the innerhousing 14.

What is claimed is:
 1. A system for heating optical members, the systemcomprising: a thermally-conductive inner housing defining an interiorvolume for receiving an optical member to be heated; athermally-insulative outer housing at least partially containing thethermally-conductive inner housing; and a heating structure disposedoutside the inner housing and configured to provide heat through thethermally-conductive inner housing and into the interior volume definedby the inner housing wherein at least one of the inner housing and theouter housing has substantially orthotropic thermal conductivity.
 2. Thesystem of claim 1 wherein the inner housing is configured such that aninner surface defining the interior volume has a substantially uniformtemperature in response to the inner housing receiving the heat providedby the heating structure.
 3. The system of claim 2 wherein the innerhousing is configured to define the interior volume to be axi-symmetric.4. The system of claim 1 further comprising a controller coupled to theheating structure and configured to control the heating structure suchthat the member disposed in the interior volume is heated substantiallywithout being plastically deformed.
 5. The system of claim 4 wherein thecontroller is configured to control the heating structure such that aresolved shear stress of a CaF₂ optical member disposed in the interiorvolume does not exceed about 0.5 e^((990/T)) MPa where T is averagetemperature of the member in Kelvin.
 6. A system for heating opticalmembers, the system comprising: a thermally-conductive inner housingdefining an interior volume for receiving an optical member to beheated, a thermally-insulative outer housing at least partiallycontaining the thermally-conductive inner housing; and a heatingstructure disposed outside the inner housing and configured to provideheat through the thermally-conductive inner housing and into theinterior volume defined by the inner housing; wherein a portion of theouter housing in contact with and supporting the inner housing has athermal conductivity different than at least one other portion of theouter housing.
 7. The system of claim 1 wherein an inner boundary of theouter housing is disposed in contact with substantially an entire outerboundary of the inner housing.
 8. The system of claim 7 wherein theinner housing and at least a portion of the outer housing are anintegral structure, with the inner housing and the at least a portion ofthe outer housing being layers of the integral structure with differentthermal conductivity.
 9. The system of claim 1 wherein the inner housingcomprises at least one of high-thermal-conductivity graphite andhigh-thermal-conductivity carbon.
 10. The system of claim 9 wherein theinterior volume is cylindrical, the inner housing has substantiallyorthotropic thermal conductivity with different directions in the innerhousing having relatively higher and lower thermal conductivities, andwherein at least some of the directions of relatively higher thermalconductivity of the inner housing are parallel with inner surfaces ofthe inner housing.
 11. The system of claim 9 wherein the interior volumeis cylindrical, the inner housing has substantially orthotropic thermalconductivity with different directions in the inner housing havingrelatively higher and lower thermal conductivities, and wherein at leastsome of the directions of relatively lower thermal conductivity of theinner housing are perpendicular with inner surfaces of the innerhousing.
 12. The system of claim 9 wherein the outer housing hassubstantially orthotropic thermal conductivity with different directionsin the outer housing having relatively higher and lower thermalconductivities, and wherein directions of relatively lower thermalconductivity of the outer housing are perpendicular with outer surfacesof the inner housing.
 13. The system of claim 1 wherein the innerhousing has substantially orthotropic thermal conductivity.
 14. Thesystem of claim 1 wherein the outer housing comprises at least one oflow-thermal-conductivity graphite, low-thermal-conductivity carbon,low-thermal-conductivity porous graphite, low-thermal-conductivityporous carbon, low-thermal-conductivity fibrous graphite,low-thermal-conductivity fibrous carbon.
 15. The system of claim 1wherein the outer housing has substantially orthotropic thermalconductivity.
 16. The system of claim 1 further comprising anotherthermally-conductive housing, the another thermally-conductive housingsubstantially contains the thermally-insulative outer housing.
 17. Thesystem of claim 16 wherein the another thermally-conductive housing isdisplaced from the outer housing.
 18. A system for heating opticalmembers, the system comprising: a thermally-conductive inner housingdefining an interior volume for receiving an optical member to beheated; a thermally-insulative outer housing at least partiallycontaining the thermally-conductive inner housing; and a heatingstructure disposed outside the inner housing and configured to provideheat through the thermally-conductive inner housing and into theinterior volume defined by the inner housing; wherein the inner housingdefines a plurality of interior volumes each for receiving an opticalmember to be heated.
 19. The system of claim 1 wherein the inner housinghas a substantially isotropic thermal conductivity.
 20. The system ofclaim 1 wherein the outer housing has a substantially isotropic thermalconductivity.
 21. The system of claim 1 wherein at least a portion ofthe heating structure is disposed outside the outer housing.
 22. Thesystem of claim 1 further comprising: a controller coupled to theheating structure; and a temperature sensor coupled to the controllerand configured to provide indicia of temperature of thehigh-thermal-conductivity housing to the controller; wherein thecontroller is configured to control amounts of heat provided by theheating structure in response to the indicia provided by the temperaturesensor.
 23. The system of claim 1 wherein the inner housing has asubstantially orthotropic thermal conductivity with different directionsin the inner housing having relatively higher and lower thermalconductivities, and wherein at least some of the directions ofrelatively higher thermal conductivity of the inner housing are parallelwith inner surfaces of the inner housing.
 24. The system of claim 1wherein the inner housing has a substantially orthotropic thermalconductivity with different directions in the inner housing havingrelatively higher and lower thermal conductivities, and wherein at leastsome of the directions of relatively lower thermal conductivity of theinner housing are perpendicular with inner surfaces of the innerhousing.
 25. The system of claim 1 wherein the inner housing has asubstantially orthotropic thermal conductivity with different directionsin the outer housing having relatively higher and lower thermalconductivities, and wherein at least some of the directions ofrelatively lower thermal conductivity of the outer housing areperpendicular with inner surfaces of the inner housing.